1. Field of the Invention
This invention relates to semiconductor manufacturing, and more particularly to handling and testing semiconductor wafers.
2. Background of the Related Art
The manufacture of integrated circuits (I.C.'s) begins with blank, unpatterned semiconductor wafers. These wafers undergo a number of sometimes critical process steps before being formed into the final I.C. form. A substandard wafer can affect the number of usable I.C.'s on a wafer (yield). It is therefore desirable to have a machine for testing wafers to ensure the wafers meet a customer's standards to maximize wafer yield. The testing of wafers is often accomplished by an automated process, in which robots continuously handle and test the wafers, which tends to be more efficient than manual testing and handling of wafers, i.e. an automated process is typically faster, more precise, and less contaminating than a manual one.
Characteristics of a wafer often need to be known to test the wafer. One characteristic is the orientation of a wafer, which is used to provide a standard reference against which the location and characteristics of test points may be measured. To provide an orientation and other information, wafers are manufactured with an index mark such as a flat or notch provided in the edge of the wafer. A testing or processing apparatus can position the wafer at a desired orientation or test a specific section of the wafer by referencing the index mark. In addition, there is often a need to know the location of the center of a wafer. For example, robot arms that handle wafers should be able to determine the center of a wafer so that the arm can reliably position the wafer on a testing platform. It is therefore desirable to have a mechanism for finding both the index mark and the center of a wafer for testing and manufacturing purposes.
One way to find wafer index marks and centers is to map the circumferential edge of a wafer and use the mapped data to find index marks and the center. This can be accomplished using a variety of methods. In one method, a wafer is positioned on a rotatable platform, where the edge of the wafer is positioned within or over a sensor. The platform is rotated, and the position of the edge of the wafer is detected by the sensor throughout the entire 360-degree rotation. From this edge information, notches or flats in the wafer's edge can be found, and the center of the wafer can be calculated. Once the edge is mapped, a robot arm picks up the wafer and places the wafer on a different testing platform, such as a test chuck, in the desired orientation. Testing and processing apparatus can then perform tests and processing at specific points on the wafer.
One problem with the prior art method is that the wafer must be transported from the edge mapping platform to a different test chuck for testing or processing. This introduces some possible error to the center location and orientation data of the wafer, since the center point of the wafer on the chuck may not be precisely known due to positional shifts of the wafer occurring during transport. Thus, the location of points on the wafer which are tested on the chuck may not be known accurately. In addition, testing and processing times for a semiconductor wafer are increased due to the time of transporting the wafer from an edge mapping apparatus to a testing/processing apparatus.
Another problem with the prior art handling mechanisms is that the cost of the mechanisms can be excessive. A robot mechanism is required to unload the wafers from the wafer carrier (cassette or pod), and the robot must have several degrees of freedom to access the wafers of the carrier, including a fairly large z-axis movement. Furthermore, the robot must be able to move the wafer from the carrier to an edge mapping station, and from there to a separate testing station. In addition, two separate rotatable platforms/chucks must be maintained, one for mapping and one for testing. These functions require complex and costly mechanisms that require more maintenance, which is undesirable in production environments.
What is needed is an apparatus and method that will quickly, accurately and economically move wafers, map the edge of wafers, and test the wafers in an integrated, simpler apparatus that requires less time and maintenance.